A mode-locked laser generates a train of very high-repetition rate pulses. For a laser to mode-lock, it is necessary that it oscillate in many different cavity modes at once and that these oscillations be in phase. Mode-locking is a special operation regime of lasers where an intra-cavity modulation (amplitude or phase modulator) forces all of the laser modes to operate at a constant phase, i.e., phase-locked or xe2x80x9cmode-locked,xe2x80x9d so that the temporal shape of the laser output forms a continuously repeating train of short (typically in the range of picoseconds or femtoseconds) optical pulses. A saturable absorber is a commonly utilized mode-locking device.
A saturable absorber is a material that has decreasing light absorption with increasing light intensity. Saturable absorbers show this effect at intensities typical in solid-state laser cavities. The key parameters for a saturable absorber are its wavelength range (where it absorbs), its dynamic response (how fast it recovers), and its saturation fluence (at what pulse energy per unit area it saturates). The saturable absorber has a limit on how much fluence (energy/area) it can handle; above this fluence, the saturable absorber will get damaged. Also, once the laser is mode-locked, extended exposure to intracavity laser power may damage the saturable absorber when focusing the laser beam. Conversely, if insufficient energy is provided, then the saturable absorber will cease mode-locking.
Because of the foregoing and because of other nonlinear optical effects like self phase modulation and Kerr lens mode-locking contribution, it desirable for a device and method to overdrive an oscillator such that mode-locking begins. Furthermore, such device and method would provide a feedback loop to determine the mode-lock status of the oscillator and provide an overdrive to the oscillator pump source to initiate mode-locking and then reduce the laser energy to a minimum sustainable energy level. Additionally, the laser power should be reduced to avoid double pulsing due to high intracavity power.
The present invention is directed to an apparatus and method for oscillator start-up control, and more particularly to an apparatus and method for overdriving a laser to obtain mode-lock operation.
To address the foregoing identified problem, a laser oscillator should be driven harder in the beginning for a short amount of time until the laser is in mode-locked operation and then the pump power reduced for normal operation. This can be achieved by overdriving the pump current of a pump diode during start up of the oscillator or when the oscillator stops mode-locking.
A feedback signal from the oscillator is needed which indicates if the laser is in mode-locked operation. The signal can be derived from a mode-lock detection device, such as a photodiode. The oscillator pump diode is extended in a way to ramp up the pump current for an adjustable amount of time and value whenever the feedback signals that the oscillator is not mode-locked.
In one aspect of the invention an oscillator with start-up control is comprised of a base, a lasing medium mounted on said base, a laser pumping source mounted on said base for inducing a laser beam from said lasing medium, said laser beam being directed along a path, a mode-lock detection device mounted about said base, and an overdrive circuit coupled with said mode-lock detection device and laser pumping source. By way of illustration, but not limitation, a laser pumping source may be a diode, a laser, an arc lamp, or other source.
The mode-locking device may be any device that can detect the mode-locked status of the oscillator. In one embodiment, the mode-lock detection device is a photodiode.
Various configurations of oscillators may be utilized with the present invention. In one embodiment of the invention, an oscillator has a lasing medium composed of Nd:glass. Other laser materials and crystals like Yb, Ti, Cr doped hosts and others would behave very similarly regarding the overdrive support.
A further aspect of the invention is an overdrive circuit. The overdrive circuit is configured to detect mode-lock status utilizing the mode-lock detection device. When the oscillator is not mode-locked, which may occur during initiation or normal operations of the oscillator, the overdrive current will provide a current to a pump source, for example laser pumping diode.
In one embodiment, the laser beam is formed of a continuously repeating train of short optical pulses in the range of picoseconds or femtoseconds.
In another aspect of the invention, a method for oscillator mode-locking, comprises the steps of (a) providing an overdrive current to an oscillator for a duration of time until said oscillator is mode-locked, (b) reducing the overdrive current to the laser oscillator to a normal operating current for said laser oscillator, and (c) periodically monitoring the oscillator to determine if the oscillator is mode-locked.
In yet a further aspect of the invention, steps (a)-(c) are performed if in step (c) the oscillator is not mode-locked.
In one embodiment of the invention, an error signal is generated if the oscillator is not mode-locked. A mode-lock detection device is used to determine if the oscillator is mode-locked. If the oscillator is not mode-locked, then a circuit monitoring the mode-lock status of the oscillator will generate an error signal.
For purposes of illustration, but not limitation, examples of the pulsed lasers suitable for use with the apparatus and method include Nd:glass lasers such as the IntraLase FS laser, an argon ion-pumped solid state mode-locked laser, such as the Coherent Inc. Inova (argon) and Mira (Ti:sapphire); a diode-laser-pumped solid state mode-locked laser, such as a continuous wave diode-pumped frequency-doubled YAG and mode-locked Ti:sapphire laser; and a direct diode-pumped mode-locked solid state laser.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter, which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.